Aluminosilicate glasses for ion exchange

ABSTRACT

Glass compositions that may be used to produce chemically strengthened glass sheets by ion exchange. The glass compositions are chosen to promote simultaneously high compressive stress and deep depth of layer or, alternatively, to reduce the time needed to ion exchange the glass to produce a predetermined compressive stress and depth of layer.

BACKGROUND

The disclosure relates to ion exchangeable glasses. More particularly, the disclosure relates to ion exchangeable glasses that, when ion exchanged has a surface layer that is under a compressive stress of at least about 1 GPa.

The ion exchange process is used to strengthen glass by creating a compressive stress at the glass surface by replacing of relatively large alkali ions such as K⁺ from a salt bath with smaller alkali ions such as Na⁺ in the glass. Since glasses typically fail under tension, the created compressive stress at the surface improves the glass strength. Ion exchanged glasses thus find use in various applications such as touch-screen devices, hand held electronic devices such as communication and entertainment devices, architectural and automotive components, and the like.

When strengthened by ion exchange, a glass should simultaneously be provided with high compressive stress at the surface and a deep depth of the ion exchange layer. Soda-lime glasses are difficult to chemically strengthen by ion exchange as they require long salt bath treatment times to achieve reasonable strength by ion exchange.

SUMMARY

The present disclosure provides glass compositions that may be used to produce chemically strengthened glass sheets by ion exchange. The glass compositions are chosen to promote simultaneously high compressive stress and deep depth of layer or, alternatively, to reduce the time needed to ion exchange the glass to produce a predetermined compressive stress and depth of layer.

Accordingly, one aspect of the disclosure is to provide an alkali aluminosilicate glass. The alkali aluminosilicate glass comprises from about 14 mol % to about 20 mol % Al₂O₃ and from about 12 mol % to about 20 mol % of at least one alkali metal oxide R₂O selected from the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O, wherein the alkali aluminosilicate glass is ion exchangeable.

A second aspect of the disclosure is to provide an alkali aluminosilicate glass. The alkali aluminosilicate glass comprises from about 55 mol % to about 70 mol % SiO₂; from about 14 mol % to about 20 mol % Al₂O₃; from 0 mol % to about 10 mol % B₂O₃; from 12 mol % to about 20 mol % R₂O, where R₂O is selected from the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O; from 0 mol % to about 10 mol % MgO; and from 0 mol % to about 10 mol % ZnO. The alkali aluminosilicate glass is ion exchanged and has a compressive layer extending from a surface of the alkali aluminosilicate glass into the alkali aluminosilicate glass to a depth of layer. The compressive layer is under a compressive stress of at least 1 GPa.

These and other aspects, advantages, and salient features will become apparent from the following detailed description, the accompanying drawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of compressive stress as a function of [Al₂O₃]—[R₂O];

FIG. 2 is a plot of depth of layer (DOL) as a function of [Al₂O₃]—[Na₂O];

FIG. 3 is a plot of compressive stress (CS) for a fixed depth of layer of 50 μm as a function of [MgO]/([MgO]+[CaO]) ratio;

FIG. 4 is a plot of the diffusion coefficient D_(Na-K) as a function of composition of the boroaluminosilicate series of glasses described herein;

FIG. 5 a plot of the composition dependence of isothermal diffusivity and iron redox ratio;

FIG. 6 is a plot of compressive stress (CS) of both Fe-free and Fe-containing boroaluminosilicate glasses as a function of composition;

FIG. 7 a plot of the loading and penetration depth condition of the experiment performed on iron-free boroaluminosilicate glass A117.5 in Table 6; and

FIG. 8 is a plot of compositional dependence of nanohardness (H_(nano)) at 98 mN load force for iron-containing and iron-free boroaluminosilicate glasses.

DETAILED DESCRIPTION

In the following description, like reference characters designate like or corresponding parts throughout the several views shown in the figures. It is also understood that, unless otherwise specified, terms such as “top,” “bottom,” “outward,” “inward,” and the like are words of convenience and are not to be construed as limiting terms. In addition, whenever a group is described as comprising at least one of a group of elements and combinations thereof, it is understood that the group may comprise, consist essentially of, or consist of any number of those elements recited, either individually or in combination with each other. Similarly, whenever a group is described as consisting of at least one of a group of elements or combinations thereof, it is understood that the group may consist of any number of those elements recited, either individually or in combination with each other. Unless otherwise specified, a range of values, when recited, includes both the upper and lower limits of the range as well as any ranges therebetween. As used herein, the indefinite articles “a,” “an,” and the corresponding definite article “the” mean “at least one” or “one or more,” unless otherwise specified. As used herein, the term “glass” refers to alkali aluminosilicate and/or boroaluminosilicate glasses, unless otherwise specified.

Referring to the drawings in general and to FIG. 1 in particular, it will be understood that the illustrations are for the purpose of describing particular embodiments and are not intended to limit the disclosure or appended claims thereto. The drawings are not necessarily to scale, and certain features and certain views of the drawings may be shown exaggerated in scale or in schematic in the interest of clarity and conciseness.

This disclosure relates to the general area of ion exchangeable alkali aluminosilicate glasses that are capable of—or have been strengthened by—ion exchange. The ion exchange process is used to create a compressive stress at the glass surface by replacement of relatively large alkali ions from a salt bath (e.g., K⁺) with smaller alkali ions (e.g., Na⁺) in the glass. Since glasses typically fail under tension, the compressive stress created at the surface improves the glass strength. Ion exchanged glasses thus find various applications, such as for touch-screen devices, hand held electronic devices such as communication and entertainment devices, architectural and automotive components, and the like.

Ion exchangeable glass compositions should be designed so as to simultaneously provide a high compressive stress (CS) at the surface and a deep depth of the ion exchange layer (depth of layer, or “DOL”). Soda-lime glasses are typically difficult to chemically strengthen by ion exchange, as they require long salt bath treatment times to achieve reasonable strength by such exchange.

The various glass compositions described herein could be used to produce chemically strengthened glass sheets by ion exchange. These glass compositions are chosen to promote simultaneously high compressive stress and deep depth of layer or, alternatively, reduced ion exchange time. The glass compositions described herein are not necessarily fusion formable or down drawable (e.g., fusion drawn or slot drawn), and may be produced using other forming methods known in the art; e.g., the float glass process.

The glasses described herein are ion exchangeable alkali aluminosilicate glasses comprising from about 14 mol % to about 20 mol % Al₂O₃ and from about 12 mol % to about 20 mol % of at least one alkali metal oxide R₂O selected from the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O. In some embodiments, the at least one alkali metal oxide includes Na₂O, and Al₂O₃ (mol %)-Na₂O (mol %)≧about −4 mol %.

In some embodiments, the glasses described herein, when strengthened by ion exchange, have a region that is under a compressive stress (compressive layer CS) that extends from the surface of the glass to a depth of layer (DOL) into the body of the glass. The compressive stress of the strengthened glass is at least about 1 GPa. In some embodiments, the compressive stress is at least about 1 GPa and Al₂O₃ (mol %)-Na₂O (mol %)≧about −4 mol %.

In some embodiments, the glass comprises: from about 55 mol % to about 70 mol % SiO₂; from about 14 mol % to about 20 mol % Al₂O₃; from 0 mol % to about 10 mol % B₂O₃; from 0 mol % to about 20 mol % Li₂O; from 0 mol % to about 20 mol % Na₂O; from 0 mol % to about 8 mol % K₂O; from 0 mol % to about 10 mol % MgO; and from 0 mol % to about 10 mol % ZnO. In particular embodiments, 12 mol % Li₂O+Na₂O+K₂O≦20 mol %.

In one aspect, the alkali aluminosilicate glasses are sodium aluminosilicate glasses that further comprise different types of divalent cation oxides RO, also referred to herein as “divalent metal oxides” or simply “divalent oxides” in which the silica-to-alumina ratio ([SiO₂]/[Al₂O₃]) is not fixed, but may instead be varied. These divalent metal oxides RO include, in one embodiment, MgO, ZnO, CaO, SrO, and BaO. Non-limiting examples of such compositions having the general formula (76-x) mol % SiO₂, x mol % Al₂O₃, 16 mol % Na₂O, and 8 mol % RO, in which x=0, 2.7, 5.3, 8, 10.7, 13.3, 16, 18.7, 21.3, and 24 and properties associated with each composition are listed in Tables 1, 2, and 3, for R=Mg, R=Zn, and R=Ca, respectively. Non-limiting examples of such compositions, expressed in mol % where (76-x)SiO₂-xAl₂O₃-16Na₂O-8RO, where x=0, 8, 16, and 24, and properties associated with such compositions for R=Sr and Ba are listed in Table 5. For x=16, four glasses with [MgO]/[CaO] ratios equal to 0.25, 0.67, 1.5, and 4 were also studied, in addition to glasses with K₂O-for-Na₂O substitutions and higher SiO₂ contents (Table 4). In some embodiments, these glasses are free of (i.e., contain 0 mol %) boron and boron-containing compounds, such as, for example, B₂O₃.

In other embodiments, the alkali aluminosilicate glasses described herein are boroaluminosilicate glasses comprising up to about 10 mol % B₂O₃ with varying silica-to-alumina ratios. These boroaluminosilicate glasses may, in some embodiments, be free of (i.e., contain 0 mol %) divalent metal oxides RO, such as those described hereinabove. Non-limiting examples of such boroaluminosilicate glasses having nominal compositions, expressed in mol % of: (80-y) mol % SiO₂, y mol % Al₂O₃, 15 mol % Na₂O, and 5 mol % B₂O₃, where y=0, 1, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, and 20, and associated properties are listed in Table 6.

In the glass compositions described herein, SiO₂ serves as the primary glass-forming oxide. The concentration of SiO₂ should be sufficiently high in order to provide the glass with sufficiently high chemical durability suitable for touch applications. However, the melting temperature (i.e., the 200 poise temperature) of pure SiO₂ or high-SiO₂ glasses is too high to be practical for most manufacturing processes, since defects such as fining bubbles may appear. Furthermore, when compared to every oxide except boron oxide (B₂O₃), SiO₂ decreases the compressive stress created by ion exchange.

Alumina (Al₂O₃) can also serve as a glass former in the glasses described herein. Like SiO₂, alumina generally increases the viscosity of the melt and an increase in Al₂O₃ relative to the alkalis or alkaline earths in the glass generally results in improved durability. The structural role of aluminum ions depends on the glass composition. When the concentration of alkali metal oxides [R₂O] is greater than the concentration of alumina [Al₂O₃], all aluminum is primarily found in tetrahedral coordination with the alkali ions acting as charge-balancers. For [Al₂O₃]>[R₂O], there is an insufficient amount of alkali metal oxides to charge balance all aluminum in tetrahedral coordination. However, divalent cation oxides (RO) can also charge balance tetrahedral aluminum to varying degrees. Whereas Calcium, strontium, and barium all primarily behave in a manner equivalent to two alkali ions, the high field strength magnesium and zinc ions do not fully charge balance aluminum in tetrahedral coordination, and result in the formation of five- and six-fold coordinated aluminum. Al₂O₃ generally plays an important role in ion-exchangeable glasses, since it provides or enables a strong network backbone (i.e., a high strain point) while allowing for the relatively fast diffusivity of alkali ions. As evidenced by the plot of compressive stress as a function of [Al₂O₃]—[R₂O] in FIG. 1 for the glass compositions listed in Tables 1-5 after ion exchange in technical grade KNO₃ at 410° C. for eight hours, the presence of tetrahedral aluminum promotes a high compressive stress. As seen in FIG. 1, compressive stress CS generally increases with increasing alumina content and decreasing size of the divalent cation. In the peraluminous regime, there is an advantage from having the larger divalent cations. Most likely these cations act to charge balance tetrahedral aluminum, whereas the smaller divalent cations in MgO and ZnO do not to the same extent. In glasses with excess magnesium and zinc, however, the addition of alumina decreases the depth of the compressive layer for a given ion exchange time when [Al₂O₃]>[R₂O].

Although B₂O₃ is also a glass-forming oxide, it can be used to reduce viscosity and liquidus temperature. In general, an increase in B₂O₃ of 1 mol % decreases the temperature at equivalent viscosity by 10-14° C., depending on the details of the glass composition and the viscosity in question. However, B₂O₃ can lower liquidus temperature by 18-22° C. per mol %, and thus has the effect of decreasing liquidus temperature more rapidly than it decreases viscosity, thereby increasing liquidus viscosity. Furthermore, B₂O₃ has a positive impact on the intrinsic damage resistance of the base glass. However, B₂O₃ has a negative impact on ion exchange performance, decreasing both the diffusivity and the compressive stress. For example, substitution of SiO₂ for B₂O₃ increases ion exchange performance but simultaneously increases melt viscosity.

Alkali metal oxides (Li₂O, Na₂O, and K₂O) serve as aids in achieving low melting temperature and low liquidus temperatures. However, the addition of alkali metal oxides dramatically increases the coefficient of thermal expansion (CTE) and lowers chemical durability.

The presence of a small alkali metal oxide such as Li₂O and/or Na₂O is necessary to exchange with larger alkali ions (e.g., K⁺) to perform ion exchange from a salt bath and thus achieve a desired level of surface compressive stress in the glass. Three types of ion exchange can generally be carried out: Na⁺-for-Li⁺ exchange, which results in a deep depth of layer but low compressive stress; K⁺ for-Li⁺ exchange, which results in a small depth of layer but a relatively large compressive stress; and K⁺ for-Na⁺ exchange, which results in intermediate depth of layer and compressive stress. A sufficiently high concentration of the small alkali metal oxide is necessary to produce a large compressive stress in the glass, since compressive stress is proportional to the number of alkali metal ions that are exchanged out of the glass. The presence of a small amount of K₂O generally improves diffusivity and lowers the liquidus temperature, but increases the CTE.

Divalent cation oxides RO such as, but not limited to, alkaline earth oxides and ZnO, also improve the melting behavior of the glass. With respect to ion exchange performance, however, the presence of divalent cations acts to decrease alkali metal ion mobility. The effect on ion exchange performance is especially pronounced with the larger divalent cations such as, for example, Sr²⁺ and Ba²⁺, as illustrated in FIG. 2, which is a plot of depth of layer (DOL) as a function of [Al₂O₃]—[Na₂O] for ion exchanged glasses having the composition (76-x) mol % SiO₂, x mol % Al₂O₃, 16 mol % Na₂O, and 8 mol % RO, where x=0, 2.7, 5.3, 8, 10.7, 13.3, 16, 18.7, 21.3, and 24 for R=Mg (Table 1), Zn (Table 2), and Ca (Table 3) and x=0, 8, 16, and 24 for R=Sr and Ba (Table 5). The ion exchange was performed in a molten salt bath of technical grade KNO₃ at 410° C. for 8 hours. As seen in FIG. 2, DOL generally decreases with increasing alumina content, especially for the glasses containing MgO and ZnO in the peraluminous regime. Furthermore, as seen in FIG. 1, smaller divalent cation oxides generally help the compressive stress more than larger divalent cation oxides. In the glasses described herein, the concentrations of SrO and BaO are particularly kept to a minimum.

MgO and ZnO offer several advantages with respect to improved stress relaxation while minimizing the adverse effects on alkali diffusivity. However, when the amounts of MgO and ZnO in the glass are high, these oxides are prone to forming forsterite (Mg₂SiO₄) and gahnite (ZnAl₂O₄), or willemite (Zn₂SiO₄), thus causing the liquidus temperature to rise very steeply with MgO and ZnO contents. Furthermore, there may be some advantages from having a mixture of two alkaline earth oxides, as illustrated in FIG. 3, which is a plot of compressive stress (CS) for a fixed depth of layer of 50 μm as a function of [MgO]/([MgO]+[CaO]) ratio for ion exchanged glasses having the composition 60 mol % SiO₂, 16 mol % Al₂O₃, 16 mol % Na₂O, and 8 mol % RO. The glasses were ion exchanged in a molten salt bath of technical grade KNO₃ at 410° C. for different durations. As seen in FIG. 3, compressive stress CS at 50 μm generally increases with increasing magnesia content, but there is an advantage from having a mixture of CaO and MgO in the high-MgO regime.

In addition to the oxides described above, other oxides may be added to the glasses described herein to eliminate and reduce defects within the glass. For example, SnO₂, As₂O₃, Sb₂O₃, or the like may be included in the glass as fining agents. Increasing the concentration of SnO₂, As₂O₃, or Sb₂O₃ generally improves the fining capacity, but as they are comparatively expensive raw materials, it is desirable to add no more than is required to drive gaseous inclusions to an appropriately low level.

The main forming/stabilizing cations and molecules in silicate melts include Si⁴⁺, Al, B, Fe³⁺, Ti, P, and the like. The main network modifying cations and molecules include Na⁺, K+, Ca²⁺, Mg²⁺, Fe²⁺, F⁻, Cl⁻, and H₂O, although their role in defining the structure is often controversial. Iron as Fe³⁺ (ferric iron) can be a network former with coordination number IV or V and/or a network modifier with coordination V or VI, depending on the Fe³⁺/ΣFe ratio, whereas Fe²⁺ (ferrous) iron is generally considered to be a network modifier. As both ferric and ferrous iron can be present in liquids, changes in the oxidation state of iron can affect significantly their degree of polymerization. Therefore, any melt property that depends on the number of non-bridging oxygen per tetrahedron (NBO)/T) will also be affected by the ratio Fe³⁺/ΣFe. Significant portions of Si and Al may exist in five-fold coordination at ambient pressure.

In order to explore different structural roles filled by sodium in the boroaluminosilicate glasses, ten Na₂O—B₂O₃—Al₂O₃—SiO₂ glasses with variation of the [Al₂O₃]/[SiO₂] ratio were designed to access different regimes of sodium behavior. Ten additional ten glasses having the same base composition, but doped with 1 mol % Fe₂O₃ were also prepared to study the effect of Fe₂O₃ on ion exchange properties. The compositions of these glasses are designated as x mol % Al₂O₃, 5 mol % B₂O₃, (80-x) mol % SiO₂, and 15 mol % Na₂O, where x=0, 1, 2.5, 5, 7.5, 10, 12.5, 15, 17.5, and 20, with the analyzed compositions being slightly different from the batched compositions. The original naming convention based on xAl₂O₃, as given in Table 6, is retained. As a result of this work, the different roles/effects of sodium on the network-forming cations (Si, B, and Al) have been clarified and quantified. When Na<Al, all sodium is used to charge compensate [AlO₄], and [AlO₅] groups, which are also present in the glass and act as charge compensators due to insufficient amounts of sodium in the glass. When Na>Al, sodium first charge compensates [AlO₄], and all Al is thus four-coordinated and unaffected by other compositional changes. Excess sodium can be used to convert [BO₃] to [BO₄], or create non-bridging oxygens (NBOs) on Si or B, with competition among these mechanisms.

Ion exchange experiments were conducted to obtain the effective interdiffusion coefficient D _(Na-K) between Na⁺ and K⁺ and the compressive stress (CS) in the glasses described herein. Ion exchange was carried out by immersing polished 25 mm×25 mm×1 mm glass samples in a molten salt bath of technical grade KNO₃ at 410° C. for 8 hours. Following ion exchange, the penetration depth of the potassium ions was measured using an FSM-6000 surface stress meter (FSM). The K⁺ for-Na⁺ ion exchange gives the glass surface a higher refractive index than the interior; i.e., the surface acts as a waveguide. This is utilized in the FSM instrument to measure the saturation depth of the refractive index profile, which corresponds to the diffusion depth of potassium. A total of eight FSM measurements were performed on each sample (using four 90° rotations per face).

The results of these ion exchange experiments reveal a decrease in alkali diffusivity with increasing [SiO₂]/[Al₂O₃] or [SiO₂]/Σ[Oxi] where Σ[Oxi]=[SiO₂]+[Al₂O₃]+[B₂O₃]+[Fe₂O₃]+[As2O₃] ratios for both iron-containing and iron-free glasses. FIG. 4 is a plot of the diffusion coefficient D_(Na-K) as a function of composition of the boroaluminosilicate series of glasses described herein. The data plotted in FIG. 4 show that the roles of sodium and boron change as the [SiO₂]/[Al₂O₃] ratio changes. This trend may be ascribed to two factors. First, the structural role of sodium in influencing sodium diffusion depends on the [SiO₂]/[Al₂O₃] ratio. For high Al₂O₃ contents, Na⁺ is used for charge compensation of four-fold aluminum species. In this case, the diffusion of Na⁺ is relatively fast, as shown in FIG. 5, which is a plot of the composition dependence of isothermal diffusivity (K⁺ for-Na⁺ effective interdiffusion coefficient (D _(Na-K))), as determined by ion exchange experiments at 410° C., and the iron redox state, as determined by ⁵⁷Fe Mössbauer spectroscopy. This fast diffusion rate of Na may be because Na⁺ is not a rigid part of the glass network. In the low Al₂O₃ composition region, some of the sodium ions create NBOs bonded with Si—O or B—O, and these sodium ions are less mobile. Secondly, the differences in boron speciation and chemical composition lead to differences in atomic packing of the glass networks. The network becomes more densely packed with increasing [SiO₂]/[Al₂O₃] ratio, and this contributes to the lowering of the alkali diffusivity. FIG. 5 also reveals that the alkali diffusivity is greater in iron-free glasses than in iron-containing glasses. Furthermore, the difference in alkali diffusivity between iron-free and iron-containing glasses decreases with increasing [SiO₂]/[Al₂O₃] ratio while at the same time the [Fe³⁺]/[Fe]_(total) ratio increases (see the second y-axis in FIG. 5)). Therefore, Fe²⁺ is a greater hindrance to alkali diffusivity than Fe³⁺. In other words, there is little or no decrease in alkali diffusivity when iron is present as Fe³⁺. The impact of iron on alkali diffusivity may be ascribed to two factors. First, there is competition between cations for the charge compensation of AlO₄ ⁻ and BO; units. It has been shown that Fe²⁺ can charge compensate AlO₄ ⁻ units in aluminosilicate glasses, even though alkali ions are more efficient charge compensators than Fe²⁺. It is therefore possible that some Fe²⁺ ions can compete with Na+ ions for charge compensating AlO₄ ⁻ (and possibly also BO₄ ⁻), which could cause some of the sodium ions to create NBOs on tetrahedral silicon or trigonal boron. According to the discussion above, this will lower the alkali diffusivity. Second, the presence of relatively slowly moving divalent cations lowers the mobility of the fast moving monovalent alkali cations. Fe²⁺ ions play a role as network-modifiers in the glass network, and may therefore be blocking the diffusion paths of the fast moving Na⁺ ions (similar to the impact of alkaline earth ions on alkali diffusivity). On the other hand, Fe³⁺ ions play a more network-forming role in the network, and they are therefore not occupying sites that Na⁺ ions would use for diffusion.

FIG. 6 is a plot of compressive stress (CS) of both Fe-free and Fe-containing boroaluminosilicate glasses as a function of composition (i.e., [Al₂O₃]—[Na₂O]). CS was measured by FSM on the annealed samples, which were chemically strengthened in a molten salt bath of technical grade KNO₃ salt bath at 410° C. for 8 hours. As shown in FIG. 6, compressive stress created by ion exchange was found to monotonically increase with increasing Al₂O₃ concentration in the boroaluminosilicate glasses. This finding is in agreement with that reported above for the sodium aluminosilicate glasses with different divalent cations. The iron-containing glasses were also found to generally have higher CS than the corresponding iron-free glasses, particularly in the peralkaline regime.

Additionally, eight hardness measurements using the nano-indentation technique for each composition were also performed on some of the glasses described herein. The hardness values reported in Table 6 were calculated from indentation depths ranging from 598 nm to 998 nm. FIG. 7 is a plot of the loading and penetration depth condition of the experiment performed on sample Al17.5, which is listed in Table 6. The compositional dependence of nano-hardness (H_(nano)) at 98 mN load force for both iron-containing and iron-free boroaluminosilicate glasses is plotted in FIG. 8. The gray and black solid symbols in FIG. 8 represent glasses before and after ion exchange, respectively, which were ion exchanged at 410° C. for 8 hours in a technical grade KNO₃ molten salt bath. The nano-indentation hardness technique does not reveal large differences in hardness for the iron-free and iron-containing glasses, neither before nor after being chemically strengthened in the KNO₃ salt bath at 410° C. for 8 hours. In some embodiments, the glasses described herein have a nanohardness of at least about 7 GPa after ion exchanged. Nonetheless, the ion exchanged peraluminous (Al>Na) glass end members exhibit a systematic increase in nano-hardness of about 1.5 GPa compared to glasses without a chemically strengthened surface. The peralkaline (Al<Na) ion exchanged end members also show an increase in nano-hardness when compared with glasses without chemically strengthened surfaces, but the difference is only about 0.5 GPa. This may be due to the lower compressive stresses created in these peralkaline compositions (FIG. 6).

While typical embodiments have been set forth for the purpose of illustration, the foregoing description should not be deemed to be a limitation on the scope of the disclosure or appended claims. Accordingly, various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the spirit and scope of the present disclosure or appended claims.

TABLE 1 Examples of ion-exchangeable glass compositions containing MgO. The compressive stress (CS) and depth of layer (DOL) were obtained as a result of treatment of annealed samples at 410° C. for 16 hours in technical grade KNO₃. Composition (mol %) 1 2 3 4 5 6 7 8 9 10 SiO₂ 75.83 73.70 70.88 68.07 65.33 62.77 59.92 56.62 54.64 52.02 A1₂O₃ 0.07 2.71 5.32 7.99 10.72 13.31 15.98 18.63 21.33 23.97 Na₂O 15.63 15.73 15.68 15.71 15.74 15.78 15.77 15.55 15.78 15.82 K₂O MgO 8.11 7.62 7.88 7.98 7.95 7.90 8.08 8.94 7.99 7.93 ZnO CaO 0.19 0.07 0.09 0.09 0.09 0.08 0.09 0.09 0.09 0.09 SrO BaO SnO₂ 0.16 0.16 0.16 0.16 0.16 0.15 0.16 0.16 0.16 0.16 Properties 1 2 3 4 5 6 7 8 9 10 Anneal Pt (C): 507 541 578 617 652 683 698 712 723 732 Strain Pt (C): 462 495 530 568 601 632 648 662 674 685 Density 2.415 2.424 2.437 2.448 2.461 2.47 2.49 2.501 2.513 2.527 (g/cm{circumflex over ( )}3): CTE (×10{circumflex over ( )}- 87.4 86.2 86.1 85.8 84.1 82.80 80.50 76.2 74.5 70.3 7/C): Softening Pt 708.7 748.7 791.8 836.1 875.1 909.40 924.90 936.8 939.7 938.6 (C): 24-h Liquidus 985 no no no 1180 >1250 1250 >1385 >1385 >1385 (C): devit devit devit Primary Devit tridymite forsterite forsterite forsterite forsterite unknown unknown Phase: Liquidus Visc 30364 21433 11390 <1517 <1010 <813 (Poise): Poisson's 0.212 0.215 0.204 0.219 0.206 0.22 0.22 0.215 0.225 0.22 Ratio: Shear 27.59 28.06 28.70 29.22 30.00 30.54 31.16 31.92 32.66 33.38 Modulus (GPa): Young's 66.89 68.20 69.09 71.24 72.36 74.44 76.16 77.59 80.03 81.46 Modulus (GPa): Refractive 1.4971 1.4992 1.5011 1.5034 1.5061 1.5090 1.5123 1.5160 1.5196 1.5234 Index: SOC 28.49 28.66 28.54 28.67 28.75 28.60 28.36 27.96 27.5 27.14 (nm/cm/MPa): CS (MPa): 128 441 663 876 1062 1154 1192 1166 1124 1056 DOL (μm): 44.19 48.63 47.90 46.45 46.33 43.88 39.59 32.26 24.96 19.21

TABLE 2 Examples of ion-exchangeable glass compositions containing ZnO. The compressive stress (CS) and depth of layer (DOL) were obtained as a result of treatment of annealed samples at 410° C. for 16 hours in technical grade KNO₃. Composition (mol %) 11 12 13 14 15 16 17 18 19 20 SiO₂ 76.35 73.53 71.04 68.24 65.50 62.91 60.03 57.34 54.70 52.01 Al₂O₃ 0.02 2.72 5.34 8.03 10.74 13.38 16.02 18.80 21.36 24.05 Na₂O 15.42 15.61 15.61 15.64 15.57 15.74 15.62 15.79 15.66 15.74 K₂O MgO ZnO 8.06 7.98 7.86 7.93 8.03 7.82 8.17 7.92 8.12 8.04 CaO SrO BaO SnO₂ 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 Properties 11 12 13 14 15 16 17 18 19 20 Anneal Pt (C): 513 544 577 609 639 658 673 684 696 708 Strain Pt (C): 467 497 528 562 589 609 625 635 647 660 Density 2.541 2.558 2.566 2.570 2.581 2.585 2.600 2.611 2.623 2.636 (g/cm{circumflex over ( )}3): CTE (×10{circumflex over ( )}- 86.4 85.6 85.7 84.8 83.4 81.8 78.3 75.8 71.7 68.9 7/C): Softening Pt 706 742 779 819 857 885 902 909 910 915 (C): 24-h Liquidus 1040 no devit 870 935 no 1070 1370 >1390 >1390 >1390 (C): devit Primary Devit Tridy Albite Albite Unknown Spinel Unknown Unknown Unknown Phase: mite Liquidus Visc 191253 87234 19996 (Poise): 8634 4 2 0 1849 <1231 <909 <10 Poisson's 0.218 0.214 0.216 0.217 0.223 0.22 0.23 0.228 0.226 0.24100 Ratio: Shear 27.02 27.78 28.65 29.04 29.50 30.21 30.77 31.53 32.31 32.96 Modulus (GPa): Young's 65.81 67.48 69.00 70.69 72.13 73.79 75.50 77.41 79.22 81.82 Modulus (GPa): Refractive 1.508 1.514 1.516 1.534 1.524 Index: 0 1.5102 1.5123 1 1 5 1.5215 7 1.5286 1.5325 SOC 33.08 32.94 32.99 32.74 32.22 31.65 31.01 30.36 29.67 29.10 (nm/cm/MPa): CS (MPa): 467 659 872 1070 1134 1186 1165 1123 1023 DOL (μm): 50.01 49.50 47.01 45.57 44.24 39.31 32.32 25.63 19.65

TABLE 3 Examples of ion-exchangeable glass compositions containing CaO. The compressive stress (CS) and depth of layer (DOL) were obtained as a result of treatment of annealed samples at 410° C. for 16 hours in technical grade KNO₃. Composition (mol %) 21 22 23 24 25 26 27 28 29 30 SiO₂ 75.88 73.19 70.73 68.08 65.20 62.58 59.83 57.18 54.26 51.82 Al₂O₃ 0.03 2.71 5.30 8.02 10.72 13.29 16.01 18.71 21.34 23.97 Na₂O 15.72 15.76 15.78 15.72 15.77 15.80 15.79 15.68 15.70 15.81 K₂O MgO 0.10 0.10 0.11 0.09 0.12 0.12 0.13 0.13 0.13 0.13 ZnO CaO 8.11 8.10 7.91 7.92 8.03 8.05 8.08 8.15 8.40 8.11 SrO BaO SnO₂ 0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.15 0.16 0.16 Properties 21 22 23 24 25 26 27 28 29 30 Anneal Pt (C): 525 548 567 591 619 647 678 710 738 756 Strain Pt (C): 483 505 524 547 574 601 630 661 690 709 Density 2.474 2.485 2.491 2.499 2.509 2.52 2.52 2.528 2.537 2.547 (g/cm{circumflex over ( )}3): CTE (×10{circumflex over ( )}- 92.5 90.9 89.4 88.2 87.7 87.00 85.40 82.8 80.2 77.3 7/C): Softening Pt 700.2 725.1 749.9 779.5 811 845.00 882.10 921.5 (C): 24-h Liquidus 990 900 990 1070 1250 1250 1155 1245 1300 1295 (C): Primary Devit tridymite devitrite devitrite devitrite anorthite anorthite nepheline nepheline unknown unknown Phase: Liquidus Visc 10657 2165 3704 29476 10457 4955 4573 (Poise): Poisson's 0.212 0.212 0.223 0.221 0.223 0.23 0.22 0.237 0.238 0.221 Ratio: Shear 28.78 29.28 29.70 30.10 30.53 30.91 31.25 31.62 32.16 32.82 Modulus (GPa): Young's 69.75 71.00 72.62 73.48 74.66 75.70 76.47 78.24 79.64 80.17 Modulus (GPa): Refractive 1.5119 1.5138 1.5150 1.5166 1.5183 1.5198 1.5218 1.5235 1.5259 1.5292 Index: SOC 27.33 27.41 27.5 27.49 27.36 27.39 27.45 27.35 27.11 26.71 (nm/cm/MPa): CS (MPa): 381 601 738 911 1037 1123 1152 1139 1068 DOL (μm): 25.18 24.85 25.90 26.98 28.10 27.81 25.83 22.11 18.07

TABLE 4 Examples of ion-exchangeable glass compositions containing a mixture of MgO and CaO. The compressive stress (CS) and depth of layer (DOL) were obtained as a result of treatment of annealed samples at 410° C. for 16 hours in technical grade KNO₃. Composition 40 (mol %) 31 32 33 34 35 36 37 38 39 SiO₂ 59.85 59.81 59.73 59.91 60.11 59.93 60.08 60.00 61.92 63.96 A1₂O₃ 15.97 16.02 16.00 15.99 15.96 15.98 15.99 15.99 15.18 14.39 Na₂O 15.85 15.70 15.75 15.79 15.82 15.83 14.67 13.86 15.00 14.21 K₂O 1.07 1.87 MgO 1.65 3.41 5.14 5.76 6.39 7.33 5.65 5.74 5.46 5.14 ZnO CaO 6.52 4.90 3.23 2.40 1.57 0.78 2.38 2.39 2.28 2.14 SrO BaO 0.16 0.16 0.15 0.16 0.16 0.16 0.16 0.16 0.16 0.15 Properties 31 32 33 34 35 36 37 38 39 40 Anneal Pt 677 681 683 686 692 695 684 685 689 693 (C): Strain Pt (C): 629 632 633 637 642 645 635 635 639 641 Density 2.516 2.507 2.5 2.496 2.492 2.49 2.50 2.495 2.485 2.472 (g/cm{circumflex over ( )}3): CTE (×10{circumflex over ( )}- 84.5 82.9 82.4 81.8 81.7 81.60 84.10 85.2 78.9 76.5 7/C): Softening Pt 889 901 910 914 919 922 917 920 923 934 (C): 24-h Liquidus 1150 1160 1150 1160 1190 1240 1160 1185 1165 1160 (C): Primary Devit nepheline nepheline nepheline forsterite forsterite forsterite forsterite forsterite forsterite forsterite Phase: Liquidus Visc 37523 49496 58629 53212 13399 58464 38735 63867 98439 (Poise): Poisson's 0.229 0.225 0.222 0.229 0.226 0.225 0.227 0.227 0.22 0.216 Ratio: Shear 31.2 31.3 31.4 31.3 31.3 31.3 31.5 31.5 31.1 31.0 Modulus (GPa): Young's 76.8 76.6 76.6 77.0 76.7 76.7 77.2 77.3 76.0 75.3 Modulus (GPa): Refractive 1.5198 1.5177 1.5159 1.5151 1.5138 1.5132 1.5149 1.5149 1.5128 1.5102 Index: SOC 27.61 27.79 27.94 28.04 28.14 28.16 27.93 27.94 28.38 28.68 (nm/cm/MPa): CS (MPa): 1168 1196 1210 1212 1202 1197 1140 1088 1172 1136 DOL (μm): 29.31 31.62 33.19 34.60 37.27 38.52 41.24 45.69 36.11 38.15

TABLE 5 Examples of ion-exchangeable glass compositions containing SrO or BaO. The compressive stress (CS) and depth of layer (DOL) were obtained as a result of treatment of annealed samples at 410° C. for 16 hours in technical grade KNO₃. Composition (mol %) 41 42 43 44 45 46 47 48 SiO2 76.26 67.91 60.22 52.01 75.99 67.70 60.27 51.86 Al2O3 0.03 7.96 15.94 23.96 0.03 8.07 15.98 24.05 Na2O 15.58 15.90 15.72 15.87 15.62 15.89 15.60 15.88 K2O MgO ZnO CaO SrO 7.99 8.09 7.98 8.02 BaO 8.21 8.17 7.99 8.05 SnO2 0.14 0.14 0.14 0.15 0.16 0.17 0.16 0.16 Properties 41 42 43 44 45 46 47 48 Anneal Pt (C): 501 565 659 773 470 539 633 774 Strain Pt (C): 460 520 610 721 431 496 585 725 Density (g/cm{circumflex over ( )}3): 2.605 2.635 2.646 2.66 2.732 2.75 2.75 2.764 CTE (×10{circumflex over ( )}-7/C): 96.8 91.2 90 81 100.7 93.90 87.60 86.3 Softening Pt (C): 676 751 867 648 722 849 24-h Liquidus (C): 990 1000 1165 >1390 980 860 1240 1360 Primary Devit Tridymite Unknown Unknown Unknown Tridymite Albite Unknown Unknown Phase: Liquidus Visc 7898 33812 19965 <1645 5587 517914 10 3545 (Poise): Poisson's Ratio: 0.225 0.23 0.229 0.232 0.228 0.23 0.23 Shear Modulus 27.65 29.59 31.10 32.28 26.53 28.66 31.67 (GPa): Young's Modulus 67.77 72.77 76.46 79.57 65.16 70.66 77.92 (GPa): Refractive Index: 1.5150 1.5212 1.5251 1.5310 1.5242 1.5296 1.5326 1.5370 SOC 26.82 26.62 26.73 25.92 25.43 25.44 25.74 26.35 (nm/cm/MPa): CS (MPa): 695 1137 1093 571 1053 1040 DOL (μm): 21.46 20.43 18.41 17.40 15.71 18.18

TABLE 6 Analyzed compositions and selected properties of boroaluminosilicate glasses in which the ratio [SiO₂]/[Al2O₃] was modified. The iron redox ratio was determined by ⁵⁷Fe Mössbauer spectroscopy on iron-containing glasses. log Hardness Glass Chemical composition (mol %) CS N4 Diffusivity [Fe³⁺]/[Fe]₁ H_(nano) ID SiO₂ Al₂O₃ Na₂O B₂O₃ Fe₂O₃ As₂O₃ (MPa) (at %) (cm²/s) _(ot) (at %) (Gpa) Al0* 79.4 0.3 14.6 4.9 0.9 0 390 n/a −10.63 n/a 7.25 Al1* 78.9 0.7 14.5 4.9 0.9 0 421.7 n/a −10.67 n/a 7.21 Al2.5* 77.4 2.2 14.6 4.9 0.9 0 451 n/a −10.74 94 7.74 Al5* 74.7 4.7 14.6 5 1 0 558.6 n/a −10.72 92 7.91 Al7.5* 71.8 7.6 14.7 4.9 1 0 688.3 n/a −10.59 90 7.85 Al10* 68.9 10.3 14.8 5 1 0 789 n/a −10.46 81 7.85 Al12.5* 67.1 12.6 14.3 5 1 0 906.8 n/a −10.26 78 7.78 Al15* 64.1 15.6 14.3 5 1 0 995 n/a −10.16 76 7.46 Al17.5* 62.3 17.9 13.7 5.1 0.9 0 1073.3 n/a −10.35 n/a 7.30 Al20* 61.1 19.4 13.6 5 0.9 0 1041.4 n/a −10.54 n/a 7.27 Al0 80.1 0.2 14.8 4.8 0 0.2 364.8 95 −10.46 n/a 7.08 Al1 79.4 1.2 14.5 4.9 0 0.1 400.4 92 −10.55 n/a 7.19 Al2.5 78.8 2 14.4 4.7 0 0.1 370.6 90 −10.57 n/a 7.45 Al5 78.1 4 13.6 4.2 0 0.1 445.8 87 −10.65 n/a 7.57 Al7.5 76.9 5.7 13 4.3 0 0.1 557 77 −10.64 n/a 8.03 Al10 75.9 7.5 12.3 4.3 0 0.1 602.1 68 −10.52 n/a 7.92 Al12.5 72 10.4 13.1 4.4 0 0.1 760.5 39 −10.31 n/a 7.92 Al15 69.2 12.7 13.5 4.6 0 0.1 869 17 −10.12 n/a 7.79 Al17.5 63 17.2 14.7 5 0 0.1 1058.9 0 −10.17 n/a 7.27 Al20 60.5 19.6 14.7 5 0 0.1 1018 0 −10.42 n/a 7.35 

1-20. (canceled)
 21. An alkali aluminosilicate glass comprising: from about 16 mol % to about 20 mol % Al₂O₃, and from about 15 mol % to about 20 mol % of at least one alkali metal oxide R₂O selected from the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O, wherein R₂O includes 15 mol % to 20 mol % Na₂O, Al₂O₃ (mol %)-Na₂O (mol %)≧−4 mol %, and the alkali aluminosilicate glass is ion exchangeable.
 22. The alkali aluminosilicate glass of claim 21, wherein the alkali aluminosilicate glass comprises: from about 55 mol % to about 70 mot % SiO₂; from about 16 mol % to about 20 mol % Al₂O₃; from 0 mol % to about 10 mol % B₂O₃; from 0 mol % to about 5 mol % Li₂O; from 15 mol % to about 20 mol % Na₂O; from 0 mol % to about 5 mol % K₂O; from 0 mol % to about 10 mol % MgO; and from 0 mol % to about 10 mol % ZnO.
 23. The alkali aluminosilicate glass of claim 21, wherein 15 mol %≦Li₂O+Na₂O+K₂O≦20 mol %.
 24. The alkali aluminosilicate glass of claim 21, further comprising at least one divalent metal oxide.
 25. The alkali aluminosilicate glass of claim 24, wherein the at least one divalent metal oxide is at least one of MgO, CaO, BaO, SrO, and ZnO.
 26. The alkali aluminosilicate glass of claim 25, wherein the alkali aluminosilicate glass contains 0% mol % B₂O₃.
 27. The alkali aluminosilicate glass of claim 21, wherein the alkali aluminosilicate glass is free of divalent metal oxides.
 28. The alkali aluminosilicate glass of claim 21, wherein the alkali aluminosilicate glass is ion exchanged and has a compressive layer extending from a surface of the alkali aluminosilicate glass into the alkali aluminosilicate glass to a depth of layer.
 29. The alkali aluminosilicate glass of claim 28, wherein the compressive layer is under a compressive stress of at least 1 GPa.
 30. The alkali aluminosilicate glass of claim 29, wherein the compressive layer is under a compressive stress of at least 1.1 GPa.
 31. The alkali aluminosilicate glass of claim 28, wherein the alkali aluminosilicate glass has a nanohardness of at least 7 GPa.
 32. The alkali aluminosilicate glass of claim 21, further comprising at least one fining agent.
 33. The alkali aluminosilicate glass of claim 32, wherein the fining agent comprises at least one of SnO₂, As₂O₃, and Sb₂O₃.
 34. The alkali aluminosilicate glass of claim 21, wherein the alkali aluminosilicate glass is free of divalent metal oxides other than MgO and ZnO.
 35. The alkali aluminosilicate glass of claim 21, wherein the alkali aluminosilicate glass has a liquidus viscosity in a range of at least about 20 kiloPoise to 64 kiloPoise.
 36. The alkali aluminosilicate glass of claim 21, wherein Na₂O (mol %)>Al₂O₃ (mol %).
 37. The alkali aluminosilicate glass of claim 21, further comprising Fe₂O₃.
 38. The alkali aluminosilicate glass of claim 37, comprising about 1 mol % Fe₂O₃.
 39. The alkali aluminosilicate glass of claim 21, wherein the Al₂O₃ is present in tetrahedral configuration.
 40. A touch-screen device or hand held electronic device comprising the alkali aluminosilicate glass of claim
 21. 